The present invention relates generally to multi-colour display systems, and more particularly to a method and apparatus for providing colour-correction of display modules of a type which may be used in multi-colour display systems. The invention is especially suited for use with three-colour modules. Each module may be constructed from three light-emitting elements such as light emitting diodes (LEDs) and may form one pixel of a multi-colour display system. It will be convenient to describe the method and apparatus in relation to that application. However, it should be understood that the invention is not thereby restricted to that application.
Differences in intensity and colour of individual LEDs, having the same nominal colour, arise from variations in their manufacturing processes. These differences can produce noticeable unevenness in colour-rendition when groups of arrays of such LEDs are used in multi-pixel displays: for example, in a video-display panel.
The theory of colour-perception by human observers is well-established [1]-[4] and is usually described in relation to the Commission Internationale de I""Eclairage (CIE) chromaticity diagram, which enables the colour result to be determined for additive combination of pure spectral colours or of colours that are already impure. The radiation from a typical LED is xe2x80x9cimpurexe2x80x9d because the light it emits is distributed over a range of wavelengths. The wavelength-distribution, or spectrum, is also variable from one LED to another LED even when the LEDs are from the same manufacturing process.
In the CIE chromaticity diagram, represented in summary form in FIG. 1, the range of colours that can be produced by additive combination of three primary colour sources is bounded by a triangle whose vertices lie at points representing the colours of the primary-sources. In order to produce the widest range of colours from three primary colour sources, the sources should be nominally red, green and blue, and should lie at points which enclose as much of the visible colour range as possible. Fortunately, a restricted range of colours is acceptable to most observers: for example, as used in colour television-receiver displays. FIG. 1 includes two solid-line triangles. Each triangle encloses a restricted range of colours obtainable by combining light-sources whose chromaticities lie at its vertices.
A colour display module can be constructed using three or more LEDs whose colours are nominally red, blue and green. An example of such a module is disclosed in U.S. Pat. No. 4,992,704. In relation to the CIE chromaticity diagram, a particular module can represent a range of colours defined by a triangle whose vertices are the actual colours of its component LEDs. Control of colour representation within the obtainable range may be achieved by adjusting the electrical current passing through each individual LED within a module.
The available range of colours will vary from one module to another, because of the differing spectra emitted by LEDs whose colour is nominally identical. Furthermore, the intensity of emitted light varies for nominally identical LEDs carrying the same current. In order to achieve consistent colour rendition across an array of LED pixel modules, both colour and intensity variations need to be reduced, relative to variations arising from the LED manufacturing process.
The present invention provides a method and apparatus for correcting the intensities and/or hues of primary colours in a multi-colour display module containing light sources, such as LEDs. The invention may allow display modules having variable intensity and hue to be assembled into arrays with relatively consistent intensity and colour rendition.
The principle of the present invention lies in the recognition that each nominally primary colour may be adjusted by the addition of small proportions of one or more of the other primary colours. Typically, three primary colours are employed, being red, green and blue, but this need not necessarily be the case. The adjusted primary colours will hereafter be termed xe2x80x9ccorrectedxe2x80x9d primary colours; and colours emitted by individual sources (eg. LEDs) prior to adjustment, will be termed xe2x80x9cuncorrectedxe2x80x9d primary colours. In terms of the CIE chromaticity diagram, the corrected colours will lie within a triangle whose vertices are defined by the uncorrected colours. For colour-consistency across a multiplicity of display modules, the same corrected colours should lie within an achievable range for every module. Referring to FIG. 1, each solid-line triangle may be considered to represent the range of colours obtainable from a three-colour LED display module. The two modules differ in the chromaticity of their uncorrected primary colours. FIG. 1 exaggerates the difference, for the sake of clarity. The range of colours that both modules can represent is represented by the dashed-line triangle, whose vertices are encircled. For these two modules, the corrected primary colours may lie at the vertices of the dashed-line triangle, or within it.
In the CIE chromaticity diagram, the corrected primary colours for a multiplicity of display modules may lie at or within the vertices of a triangle which contains the range of colours that can be produced by every member of the multiplicity of modules. In order that arrays of such modules may display the widest possible range of colours, it is desirable for this triangle to be as large as possible. The corrected primary colours should as far as possible, be chosen to approximate pure primary colours: red, green and blue. The extent to which the corrected primary colours can approximate the desired primary colours can be determined from LED manufacturers"" specifications and/or from empirical measurements performed on individual LED modules.
The corrected primary colours can be used to produce displays of variable colour and brightness by combining the corrected colours with different intensities. This is not unlike the production of variable-colour displays from pure additive primary colours, except that the range of colours able to be produced is reduced, firstly due to inherent impurity of the LED emission spectra, and secondly due to the mechanism of colour-correction proposed by the present invention.
Typically, but not necessarily, the intensities of corrected primary colours may be set by adjusting the proportion of time that each corrected primary colour is emitted. The time-proportion may be set by pulse-width modulation of LED currents, preferably at a repetition rate that is sufficient to prevent observable flicker. In correcting a particular primary colour, a main current may pass through an LED of the same nominal colour, and correction currents may be applied to LEDs of other colours, either concurrently or during part of a repetitive cycle.
The currents required for comparable light intensity may differ substantially for different colour LEDs. For this reason, it is conceivable that the correction current applied to one LED may exceed the main current in another. Therefore reference to xe2x80x9cmainxe2x80x9d and xe2x80x9ccorrectionxe2x80x9d currents should not be interpreted as a reference to xe2x80x9clargexe2x80x9d and xe2x80x9csmallxe2x80x9d currents, except in relation to currents in an LED having a single colour.
The principle of the present invention may be realised via at least two distinct techniques. The two techniques may also be applied in combination.
A first technique may involve adding to an LED display module, a circuit composed of resistors and switches so that the characteristics of individual LEDs in the module may be compensated by a choice of resistor values. Once the characteristics are compensated, the module can be treated as a colour-corrected module, and incorporated into an array of such modules without taking further account of individual LED characteristics. This technique may be referred to as hardware-based reflecting the fact that colour-correction is hard-wired into the circuit associated with each module, typically at a stage of manufacture when the module is incorporated into a display-array.
A second technique may involve adding switches and resistors to an LED display module, without attempting colour-correction at the hardware level. Instead, stored calibration data about individual LED characteristics may be relied upon to calculate required durations of LED currents. Typically, this may involve some form of computer control of switch states, with LED calibration data recorded in a memory at the time that the display module is assembled into an array of such modules. During operation, intensity and colour data, specifying a desired display or image, may be combined with calibration data to calculate required switching times, and the LEDs may be switched on and off accordingly. This technique may be referred to as software-based, although it is possible for the same principle to be applied in a hard-wired processor, or by using firm-ware. The processor may be a general-purpose computer, a microcontroller, or a digital-signal processor, depending on the size of the pixel array and the nature of its application.
According to one aspect of the present invention there is provided a colour display module having spectrally corrected sources of light for use in a display panel having a plurality of like colour display modules, said display module including:
sources of nominally red, green and blue colours, each source being subject to unwanted spectral variation;
means associated with the nominally red source for activating the nominally green and blue sources to produce a spectrally corrected red source;
means associated with the nominally green source for activating the nominally red and blue sources to produce a spectrally corrected green source; and
means associated with the nominally blue source for activating the nominally red and green sources to produce a spectrally corrected blue source;
whereby the range of colours available to be displayed by the spectrally corrected red, green and blue sources substantially does not vary from one module to another notwithstanding the spectral variation of the nominally red, green and blue sources.
According to a further aspect of the present invention there is provided a method of correcting a colour display module having sources of nominally red, green and blue colours, each source being subject to unwanted spectral variation, said module being suitable for use in a display panel having a plurality of like colour display modules, said method including the steps of:
activating the nominally green and blue sources to produce a spectrally corrected red source;
activating the nominally red and blue sources to produce a spectrally corrected green source, and
activating the nominally red and green sources to produce a spectrally corrected blue source;
whereby the range of colours available to be displayed by the spectrally corrected red, green and blue sources substantially does not vary from one module to another notwithstanding the spectral variation of the nominally red, green and blue sources.
To assist the further understanding of the invention, reference is now made to the accompanying drawings which illustrate preferred embodiments of the present invention. It is to be appreciated that these embodiments are given by way of illustration only and the invention is not to be limited by this illustration.